2-d straight-scan on imaging surface with a raster polygon

ABSTRACT

A 2-D scanning system uses a fast-rotating raster-polygon as a single scanning component to produce straight scan lines over a 2-D image surface. An approach angle of incident light beams to the raster-polygon is selected to minimize pin-cushion distortion of scan lines introduced by polygon scanning on the image surface, and a tilt angle of the rotational axis of the raster-polygon is selected to position said polygon-scanning distortion symmetrically on the image surface. In addition, scan optics are configured to generate a predetermined amount of barrel distortion of scan lines on the image surface to compensate for pin-cushion distortion introduced by polygon scanning.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/721,013, filed May 26, 2015, which application is acontinuation of U.S. patent application Ser. No. 13/245,655, filed Sep.26, 2011, now issued as U.S. Pat. No. 9,041,762. Each of theaforementioned related patent applications is herein incorporated byreference.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present invention relate generally to laser-basedimage-generating systems and, more specifically, to systems forproducing 2-D straight-line scanning on an imaging surface using araster polygon and a method of forming the same.

Description of the Related Art

In laser-based image-generating systems, a rotating polygon mirror iscommonly used to scan one or multiple laser beams across animage-generating surface, such as the light-sensitive drum of a laserphoto-copier or the phosphor screen of a laser-phosphor display. Arotating polygon mirror is a multi-faceted optical element having aplurality of reflective surfaces. A laser beam incident on one of thereflective surfaces is directed to the image-generating surface, and asthe polygon rotates, the incident laser beam sweeps across theimage-generating surface, thereby producing one line of an image on theimage-generating surface.

In some devices, a specialized rotating polygon mirror, known as araster polygon mirror, is used to produce 2-dimensional scanning oflasers across the image-generating surface. In a raster polygon mirror,each reflective surface is canted at a different angle. As with arotating polygon mirror, when the raster polygon mirror rotates, a laserbeam incident on a reflective surface of the raster polygon beam sweepsacross the image-generating surface to produce a line of an image on theimage-generating surface. However, as each subsequent reflective surfacerotates through the incident laser beam, the beam is directed to andsweeps across a different location on the image-generating surface,thereby performing 2-dimensional scanning of the laser across theimage-generating surface. Thus, a raster polygon mirror allows a laserto be scanned across a 2-dimensional surface using a single movingcomponent, thereby facilitating high-speed laser imaging technologies.

A drawback to using a raster polygon mirror for scanning lasers acrossan image-generating surface is that the lasers so directed do not followstraight lines across the image generating surface. Instead, the scanlines of the lasers have significant curvature, which greatlycomplicates image processing and timing. In addition, each distinctcanted reflective facet of a raster polygon mirror produces acorresponding distinct curvature, producing noticeable and undesirabledistortion of images produced on the image-generating surface, asillustrated in FIG. 1. FIG. 1 illustrates curved laser scan lines101-109 produced on an imaging surface 99 by a prior art laser scanningsystem using a single laser beam directed to a raster polygon mirror. Asshown, rather than being straight and parallel lines, laser scan lines101-109 are arcs. Because each of laser scan lines 101-109 is producedby a different reflective facet of the raster polygon mirror rotatingthrough the incident laser beam, and because each reflective facetproduces a different degree of distortion, each of laser scan lines101-109 is an arc with different curvature. Such distortion is primarilycaused by asymmetrical rotation properties of the different reflectivefacets and by distortion of the scan-imaging optics that focus the laseron the imaging surface. Such distortion of laser scan lines 101-109 isgenerally visible to a viewer and can result in a degraded viewingexperience.

As the foregoing illustrates, there is a need in the art for alaser-scanning system that produces straight and parallel laser scanlines on an image-generating surface using a raster-scanning polygonmirror.

SUMMARY OF THE INVENTION

One embodiment of the present invention sets forth a 2-D scanning systemthat uses a raster-polygon and specially-designed scan optics to producestraight scan-lines on an imaging surface. An approach angle of anincident light beam to the raster-scanning polygon mirror is selected tominimize pin-cushion distortion of scan lines on the imaging surface,and a tilt angle of the rotational axis of the raster-scanning polygonis selected to locate said distortion symmetrically on the imagingsurface. In addition, a scan and imaging lens is configured to generatebarrel distortion of scan lines on the imaging surface to compensate forpin-cushion distortion.

One advantage of the present invention is that a single high-speedrotational element can be used to achieve ultrafast two-dimensionalscanning of light onto an imaging surface with straight and parallelscan lines.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 illustrates curved laser scan lines produced on an imagingsurface by a prior art laser scanning system using a single laser beamdirected to a raster polygon mirror;

FIG. 2 is a schematic diagram of an imaging system configured accordingto embodiments of the invention;

FIG. 3 illustrates symmetric curved scan lines generated by araster-polygon modeling system with an optimum angle group having anapproach angle and a polygon rotation axis tilt angle and an ideal scanlens without any distortion, according to an embodiment of theinvention;

FIG. 4 illustrates laser scan lines on an imaging surface when a scanand imaging lens is configured to compensate for pin-cushion distortion,according to an embodiment of the invention;

FIG. 5 schematically illustrates one embodiment of scan optics that areconfigured with a compensatory lens-distortion function, according to anembodiment of the invention;

FIG. 6 illustrates a barrel distortion pattern generated by anembodiment of a scan and imaging lens that only includes spherical lenselements to compensate for the scanning-polygon-introduced distortionshown in FIG. 3;

FIG. 7 schematically illustrates two sets of straight scan lines on animaging surface, according to an embodiment of the invention;

FIG. 8 schematically illustrates an imaging system that includes twofolding mirrors, according to an embodiment of the invention;

FIG. 9 sets forth a flowchart of method steps for determining theconfiguration of a scan and imaging lens, according to embodiments ofthe invention;

FIG. 10 is an example of a ray-tracing scan-line diagram illustratingscan lines produced on a screen by embodiments of the invention; and

FIG. 11 schematically illustrates another embodiment of scan optics thatare configured with a compensatory lens-distortion function, accordingto an embodiment of the invention.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

FIG. 2 is a schematic diagram of a two-dimensional (2-D) scanning system100 configured according to embodiments of the invention. 2-D scanningsystem 100 is a system that generates an image on a 2-D imaging surface110 by scanning a single or multiple light beams across the surface ofimaging surface 110 in a 2-D fashion. In some embodiments, 2-D scanningsystem 100 may be a laser-based display apparatus, such as alaser-phospor display (LPD) that uses a single or multiple lasers foroptically exciting light-emitting or fluorescent materials on imagingsurface 110 to generate an image. In other embodiments, 2-D scanningsystem 100 may be an electrostatic image printing machine, in whichimaging surface 110 is a surface of a light-sensitive device. In theembodiment illustrated in FIG. 2, 2-D scanning system 100 is configuredas an LPD, and includes imaging surface 110, a laser module 120, acollimating lens 130, an approach mirror 140, a raster polygon mirror150, scan optics 160, and a control module 180 configured as shown.

Imaging surface 110 is the surface on which 2-D scanning system 100generates a still or moving image. Imaging surface 110 includesalternating regions of phosphor-containing material that, when excited,produce light of different colors, e.g., red, green, and blue, where theproduced colors are selected so that in combination said colors can formwhite light and other colors of light. The alternating regions may bestripes, dots, or other shapes. Pixel elements on imaging surface 110include three different-colored phosphor-containing regions. Individualpixel elements may be defined by the size and shape of the alternatingregions of phosphor-containing materials on imaging surface 110 and/orby the size of a focused beam 175 that excites the phosphor-containingmaterials. In one embodiment, the alternating regions ofphosphor-containing material are narrow stripes.

Laser module 120 is a laser device such as laser tower that includes oneor more laser diodes for producing excitation beams that scan acrossimaging surface 110 during operation of 2-D scanning system 100. In apreferred embodiment, multiple laser modules 120 are integrated into thesystem, forming superimposed collimated beams onto raster polygon mirror150 with different incident angles. The number of laser modules sointegrated could be 5, 10, 20, or more. For clarity, in FIG. 2 2-Dscanning system 100 is illustrated and described with a single lasermodule 120 and a single laser beam, i.e., laser beam 171. In oneembodiment, laser beam 171 is an ultraviolet (UV) laser producing lightwith a wavelength between about 400 nm and 450 nm. Laser beam 171 is amodulated light beam that is scanned across imaging surface 110 alongtwo orthogonal directions, e.g., horizontally and vertically, in araster scanning pattern to excite pixel elements on imaging surface 110and produce an image for a viewer 105. The process of directing laserbeam 171 to imaging surface 110 is described in greater detail below.

Collimating lens 130 is a single or compound lens configured tosubstantially collimate laser beam 171, thereby forming collimated beam172. Collimating lens 130 is further configured to direct collimatedbeam 172 to approach mirror 140, as shown. In embodiments in which lasermodule 120 generates multiple laser beams, collimating lens 130 may beconfigured to collimate multiple laser beams. Alternatively, in such anembodiment, collimating lens 130 may be one of an array of collimatinglenses that are each dedicated to a single input laser beam.

Approach mirror 140 is a reflective element positioned to receive anddirect collimated beam 172 to raster polygon mirror 150 as an approachbeam 173. Approach beam 173 is incident on raster polygon mirror 150 atan approach angle 141. Approach angle 141 is the angle formed betweenapproach beam 173 and the optical axis 179 of scan optics 160. It isnoted that due to the schematic nature of FIG. 2, approach angle 141 isnot displayed to scale.

In some embodiments, a configuration of 2-D scanning system 100 isselected in which approach angle 141 is minimized, since a smallerapproach angle 141 has been shown to reduce the asymmetric distortionthat is present in laser scan lines traced on imaging surface 110. Note:asymmetric distortion of scan lines on imaging surface 110 is describedbelow in conjunction with FIG. 3. A number of geometrical constraintscome into play when determining a configuration of 2-D scanning system100 in which approach angle 141 is minimized. For a particularconfiguration of 2-D scanning system 100, the magnitude of approachangle 141 selected for approach beam 173 may be determined based onheight 119 of imaging surface 110, the beam width 145 of collimated beam172, and the pupil distance 159 between raster polygon mirror 150 andscan optics 160. In some embodiments, approach mirror 140 is positionedbetween raster polygon mirror 150 and imaging surface 110, so thatapproach beam 173 is incident on the side of raster polygon mirror 150facing imaging surface 110. In such embodiments, the optical pathbetween laser module 120 and imaging surface 110 is “folded,” therebyreducing the overall depth 106 of 2-D scanning system 100 and making 2-Dscanning system 100 significantly more compact.

In some embodiments, 2-D scanning system 100 may include multiple lasermodules 120. In such embodiments, laser beams generated by the multiplelaser modules 120 may be slightly diverging rather than parallel witheach other. In such embodiments, the magnitude of approach angle 141 mayalso be selected to position a convergence point of the multiple laserbeams proximate raster polygon mirror 150 to optimize the reflection ofthe multiple laser beams of off raster polygon mirror 150.

Raster polygon mirror 150 is a multi-faceted optical element having aplurality of reflective facets 151-155, where each reflective surface isinclined at a different angle with respect to rotational axis 156 ofraster polygon mirror 150. For clarity, in FIG. 1 only five reflectivefacets 151-155 are depicted, but raster polygon mirror 150 may have morethan or fewer than five reflective facets without exceeding the scope ofthe invention. As shown, approach beam 173 reflects off reflective facet154 as a reflected beam 174, which passes through scan optics 160 and isconverted to focused beam 175.

According to some embodiments of the invention, rotational axis 156 ofraster polygon mirror 155 is positioned at a tilt angle 157 with respectto optical axis 179 of scan optics 160. Tilt angle 157 can be selectedto optimize raster-polygon-system distortion to be symmetrical.Optimization of scan line distortion on image surface 110 is describedin greater detail below in conjunction with FIGS. 3 and 4. Inembodiments in which approach mirror 140 is positioned closer to imagingsurface 110 than raster polygon mirror 150 is positioned to imagingsurface 110, tilt angle 157 is inclined toward imaging surface 110.Tilting raster polygon mirror 150 toward imaging surface 110 facilitatesdirecting reflected beam 174 through scan optics 160 and toward imagingsurface 110.

In some embodiments, scan optics 160 comprise a compound lens configuredto focus focused beam 175 on imaging surface 110 with minimal aberrationat all points on imaging surface 110. In addition, according toembodiments of the invention, scan optics 160 are configured with acompensatory lens-distortion function, so that the laser scan linesfollowed by focused beam 175 are substantially straight lines ratherthan the arc-like paths that normally result when using a raster polygonfor two-dimensional scanning. One configuration of scan optics 160 isdescribed in greater detail below in conjunction with FIG. 5. In someembodiments, the lens elements of scan optics 160 are comprised ofmaterials that are substantially transparent to a range of wavelengthsthat includes UV, visible, and infrared (IR) light, such as N-BK7 glassavailable from Schott North America of Elmsford, N.Y. In suchembodiments, reflected beam 174 may include laser beams having UV and IRwavelengths without affecting performance of 2-D scanning system 100.

Scan optics 160 are positioned from raster polygon mirror 150 by anpupil distance 159 and from imaging surface 110 by an effective focusdistance 169. Pupil distance 159 is primarily determined by approachangle 141 and the diameters of scan-optics 160. Effective focus distance169 is determined by angular-and-linear magnification of 2-D scanningsystem 100.

Control module 180 is configured to perform control functions for andotherwise manage operation of 2-D scanning system 100. Such functionsinclude receiving image data of an image to be generated and providinglaser control signals 182 to laser module 120 based on the image data.In some embodiments, control module 180 is also configured to producescanning control signals for controlling and synchronizing rasterpolygon mirror 150 and approach mirror 140, when approach mirror is amovable mirror. Control module 180 is also configured to individuallypupil distance 159 modulate power applied to the one or more lasers inlaser module 120 in order to adjust the output intensity of each lightsource as desired. Control module 180 may include one or more suitablyconfigured processors, including a central processing unit (CPU), agraphics processing unit (GPU), a field-programmable gate array (FPGA),an integrated circuit (IC), an application-specific integrated circuit(ASIC), or a system-on-a-chip (SOC), among others, and is configured toexecute software applications as required for the proper operation of2-D scanning system 100. Control module 180 may also include one or moreinput/output (I/O) devices and any suitably configured memory forstoring instructions for controlling normal and calibration operations,according to embodiments of the invention. Suitable memory includes arandom access memory (RAM) module, a read-only memory (ROM) module, ahard disk, and/or a flash memory device, among others.

In operation, 2-D scanning system 100 forms images on imaging surface110 by directing and focusing a single or multiple laser beams ontoimaging surface 110 and modulating the output intensity of the laserbeams to deliver a desired amount of optical energy to each of the threedifferent-colored phosphor-containing regions that make up each pixelelement on imaging surface 110. Each pixel element outputs light forforming a desired image by the emission of visible light created by theselective laser excitation of each phosphor-containing region in thepixel element. Consequently, modulation of the optical energy appliedto, for example, the red, green, and blue portions of each pixel elementby the incident laser beams controls the composite color and imageintensity at each image pixel element. Together, laser module 120,collimating lens 130, mirror 140, raster polygon mirror 150, and scanoptics 160 direct one or more light beams to imaging surface 110 andscan said beams both horizontally and vertically across imaging surface110 to produce a 2-D image field. For the sake of description,“vertical” with respect to imaging surface 110 in FIG. 2 is defined asparallel to arrow 118 and “horizontal” with respect to imaging surface110 is defined as perpendicular to the plane of the page.

To scan a laser beam across imaging surface 110, laser module 120generates laser beam 171, which passes through and is collimated bycollimating lens 130 to become collimated beam 172. Collimated beam 172reflects off of approach mirror 140 as approach beam 173 and is incidenton a reflective facet of raster polygon mirror 150 that is facingimaging surface 110, i.e., one of reflective facets 151-155. Reflectedbeam 174 passes through scan optics 160 to be converted to focused beam175. As raster polygon mirror 150 rotates and the reflective facetreceiving approach beam 173 moves relative to approach beam 173, focusedbeam 175 sweeps horizontally across imaging surface 110 to produce aseries of scan lines on imaging surface 110. As each subsequentreflective facet rotates through approach beam 173, focused beam 175sweeps horizontally across imaging surface 110 at a different verticalposition, since each of reflective facets 151-155 is inclined at adifferent angle with respect to rotational axis 156.

As is well-known in the art, the use of a raster-scanning polygonmirror, such as raster polygon mirror 150, as a single scanningcomponent to perform two-dimensional scanning ordinarily results inlaser scan lines on the two-dimensional surface that are significantlyand visibly distorted over an image plane rather than the preferredstraight and parallel laser scan lines. Such distortion is commonlyknown as positive or “pin-cushion” distortion. According to embodimentsof the invention, scan optics 160 are configured to compensate for saidpin-cushion distortion by optimized polygon modeling system, introducingand equal and opposite distortion, i.e., negative or “barrel”distortion, into focused beam 175. A method of determining a desiredconfiguration for scan optics 160 that compensates for pin-cushiondistortion of polygon scanning on imaging surface 110 is described belowin conjunction with FIG. 6.

FIG. 3 illustrates laser scan lines 301-309 followed by focused beam 175on imaging surface 110 when polygon-rotation-axis and approach angle areoptimized, and scan optics 160 is an ideal lens without distortion. Asshown, rather than being straight and parallel lines, laser scan lines301-309 are arcs with positive distortion. As is known in the art, thepin-cushion distortion illustrated in FIG. 3 is primarily caused by theuse of raster polygon mirror 150 to produce two-dimensional scanning oflasers onto imaging surface 110. According to embodiments of theinvention, tilt angle 157 of rotational axis 156 is selected to optimizethe positive distortion of laser scan lines 301-309 to be verticallysymmetric on imaging surface 110. Specifically, tilt angle 157 isselected so that the arcing pattern produced by scan lines 301-309 onimaging surface 110 is positioned symmetrically, i.e., the centerline320 of the “pin-cushion” is substantially aligned with the centerline ofimaging surface 110. Consequently, scan lines 301-304, which occupy thetop half of imaging surface 110, appear to be mirror images of scanlines 306-309, which occupy the bottom half of surface 110. Because thepin-cushion distortion pattern produced by scan lines 301-309 ispositioned symmetrically on imaging surface 110, a configuration of scanoptics 160 can be selected to compensate for said pin-cushion distortionusing only spherical lens elements.

In short, approach beam 173, which is directed to raster polygon mirror150 by approach mirror 140, is in the plane defined by rotational axis156 of raster polygon mirror 150 and optical axis 179 of scan optics160. This allows polygon-scanning distortion to be horizontallysymmetric on imaging surface 110 in FIG. 2. This also results inpolygon-scanning distortion that is vertically asymmetric on imagingsurface 110, since approach angle 141 is greater than 0°. The asymmetricvertical distortion can be rendered symmetric by tilting rotational axis156 toward approach mirror 140. With horizontally and verticallysymmetric polygon-scanning distortion, one can design a symmetricoptical system to substantially compensate for the residual symmetricdistortion using symmetric optical components.

FIG. 4 illustrates laser scan lines 401-409 on imaging surface 110 whenscan optics 160 are configured to compensate for pin-cushion distortion,according to an embodiment of the invention. As shown, laser scan lines401-409 are substantially straight and parallel lines rather than arcs.One of skill in the art will understand that laser scan lines 401-409are not perfectly straight and parallel lines due to a small amount ofresidual distortion that stills remains at different locations, but suchdistortion is substantially undetectable by a viewer of 2-D scanningsystem 100. The scan-line straightness error can be easily controlledwithin a range of 1/1000, e. g., 0.5 mm over a 500 mm scan line. Thus,according to embodiments of the invention, raster polygon mirror 150 canbe used to produce two-dimensional scanning of one or more lasers onto atwo-dimensional surface, i.e., imaging surface 110, without thesignificant drawback of producing visibly distorted laser scan lines.Further, when tilt angle 157 of raster polygon mirror 150 is selected tooptimize the positive distortion of laser scan lines 401-409, i.e, bypositioning the pin-cushion distortion pattern symmetrically on imagingsurface 110, scan optics 160 can be configured with only spherical lenselements to compensate for said pin-cushion distortion.

FIG. 5 schematically illustrates one embodiment of scan optics 160 thatis configured with a compensatory lens-distortion function, according toan embodiment of the invention. Because of this compensatorylens-distortion function, the laser scan lines followed by focused beam175 on imaging surface 110 are substantially straight lines rather thanthe visibly curved paths that normally result from the use of a rotatingraster polygon mirror. In addition, similar to scan lenses known in theart, scan optics 160 are configured to focus focused beam 175 on imagingsurface 110 with minimal aberration at all points on imaging surface110. In some embodiments, laser module 120 produces laser beams havingwavelengths in the UV, IR, and/or visible bands. In such embodiments,the materials of elements 501-505 are substantially transparent in thedesired wavelength band or bands. Light beams 511, 512, and 513 areshown in FIG. 5 to qualitatively illustrate the behavior of light beamsthat pass through scan optics 160 from different incident angles and areincident on imaging surface 110 at different vertical positions. Note:imaging surface 110, pupil distance 159, and effective focus distance169 are not to scale.

In the embodiment illustrated in FIG. 5, scan optics 160 comprise afive-element compound lens that includes elements 501-505, where each ofelements 501-505 has a specific function. Taken together, the functionsof elements 501-505 focus light beams 511-513 on imaging surface 110with minimal aberration and with a compensating barrel distortion thatsubstantially cancels the pin-cushion distortion that is produced byother components of 2-D scanning system 100. In the embodimentillustrated in FIG. 5, elements 501-505 are each spherical elements,which are generally more manufacturable than aspherical opticalelements. In addition, scan optics 160 are symmetrically positioned withrespect to imaging surface 110, i.e., scan optics 160 are positionedsuch that a ray passing along the optical axis 550 of scan optics 160also passes through a center point 560 of imaging surface 110. Centerpoint 560 is equidistant from top edge 561 and bottom edge 562 ofimaging surface 110 and is also equidistant from the left and rightedges (not shown) of imaging surface 110. Because scan optics 160 aresymmetrically positioned with respect to imaging surface 110, the fullclear-aperture of each component in scan optics 160 can be effectivelyused when scanning focused beam 175 on imaging surface 110. One of skillin the art will appreciate that when the full aperture of scan optics160 is used rather than a portion thereof, elements 501-505 can be morereadily manufactured, since elements 501-505 can be significantlysmaller for a given configuration of imaging system 110.

Element 501 is the first element of scan optics 160 through whichreflected beam 174 passes. Element 501 includes surfaces 501A, 501B, andis configured to generate optical power of an incident beam with minimalaberration. Element 502 includes surfaces 502A, 502B, and is configuredto compensate for on-axis aberrations introduced by Element 501. Element503 and 504 include surfaces 503A, 503B and 504A, 504B, respectively,and are configured to compensate for off-axis residual aberrationsintroduced by elements 501 and 502, such as astigmatism and fieldcurvature. Element 505 includes surfaces 505A, 505B, and is configuredmainly as a compensating distortion element that generates enoughnegative, i.e., barrel, distortion to compensate for scan-line curvatureof focused beam 175 introduced by using raster polygon mirror 150 toscan focused beam 175 on imaging surface 110. In some embodiments,element 505 is a positive, or converging, lens. In a preferredembodiment, the embodiment of scan optics 160 illustrated in FIG. 5 is aso-called “f-theta”lens, in which the position of the focused spot isdependent on the product of the focal length (“f”) of the lens and thedeflection angle (“theta”) of focused beam 175 from being normal toimaging surface 110.

Given approach angle 141, pupil distance 159, effective focus distance169, and the dimensions of imaging surface 110, one of skill in the art,upon reading the disclosure herein, can readily devise a configurationof elements 501-505 having the functionality described above. In such aconfiguration, each of elements 501-505 may vary from each other in oneor more optical characteristics, including first surface radius, secondsurface radius, element thickness, glass type, dispersion, relativeposition to adjacent elements, index of refraction, and entrance pupillocation. In some embodiments, the configuration of each of elements501-505, i.e., the above optical characteristics for elements 501-505,are determined simultaneously, since all five elements workcooperatively to ensure proper focus and barrel distortion of lightbeams 511-513 on imaging surface 110.

By way of illustration, Table 1 sets forth one embodiment of scan optics160 for a configuration of 2-D scanning system 100 in which effectivefocus distance 169 is approximately 550 mm, pupil distance 159 isapproximately 35 mm, and imaging screen is approximately 400 mm×500 mm.

TABLE 1 Index of Refraction Dispersion Surface Radius (Nd) (Vd) GlassThickness 501A −148.46 1.4970 81.61 36 12.8 501B −35.98 502A −41.251.6204 60.34 3.8 502B −158.93 503A −1149 1.6511 55.89 11.2 503B −77.29504A −49.6 1.6935 53.38 4.5 504B −186.34 505A 643.04 1.4875 70.44 13.3505B −187.24

In the embodiments described above in conjunction with FIGS. 3-5, thepin-cushion distortion of scan lines on imaging surface 110, such as thedistortion illustrated in FIG. 3, is optimized with tilt angle 157 togenerate said pin-cushion distortion symmetrically on imaging surface110. In such embodiments, an ideal scan lens is adopted, instead of realscan optics. In other embodiments, the pin-cushion distortion of scanlines on imaging surface 110 is not optimized with tilt angle 157 ofraster polygon mirror 150. Instead, an eccentric aspherical reflector ora partial lens is used to produce asymmetrical barrel distortion of scanlines that is equal-and-opposite to the asymmetrical pin-cushiondistortion of scan lines produced by 2-D scanning system 100.

For example, in one embodiment, a portion of a spherical lens system canbe used to produce a desired asymmetrical barrel distortion of scanlines to compensate for a known quantity of asymmetrical pin-cushiondistortion. FIG. 6 illustrates a barrel distortion pattern 600 generatedby an embodiment of scan optics 160 that only includes spherical lenselements. As described above in conjunction with FIG. 4, a symmetricalbarrel distortion pattern, such as barrel distortion pattern 600, may beused to compensate for the symmetrical pin-cushion distortionillustrated in FIG. 3, thereby producing straight and parallel laserscan lines 401-409 in FIG. 4. This is because centerline 320 of thepin-cushion distortion in FIG. 3 is substantially aligned with thecenterline of imaging surface 110. In contrast, in embodiments in whichthe centerline of the pin-cushion distortion is not aligned with thecenterline of imaging surface 110, and therefore is an asymmetricalpin-cushion pattern, only a portion of barrel distortion pattern 600 maybe used to produce substantially straight and parallel scan lines onimaging surface 110. Specifically, a portion 610 of barrel distortionpattern 600 can be used to compensate for the asymmetrical pin-cushiondistortion illustrated in FIG. 3. In one embodiment, to produce portion610 of barrel distortion pattern 600, scan optics 160 can be configuredas a spherical lens system in which only a fraction of the lens systemis used. In such an embodiment, the unutilized portion of the sphericallens system may be removed, such as when a larger lens system canmechanically interfere with other components in 2-D scanning system 100.Such an embodiment is illustrated in FIG. 2. In another embodiment, scanoptics 160 may instead be configured with an eccentric asphericalreflector to produce portion 610 of barrel distortion pattern 600.

In some embodiments, approach mirror 140 is configured as a movablereflective element that can be quickly and precisely rotated to adesired orientation, such as a galvanometer mirror, amicroelectromechanical system (MEMS) mirror, etc. In such embodiments,the orientation of mirror 140 alters approach angle 141 of approach beam173 to raster polygon mirror 150. As one of skill in the art willappreciate, an alteration in approach angle 141 also changes theposition on imaging surface 110 of the scan lines followed by focusedbeam 175. Thus, when approach mirror 140 is configured as a movablereflective element that can be quickly and precisely moved to multipleorientations, each orientation can direct focused beam 175 to differentportions of imaging surface 110. In this way, a single laser beam can beused to illuminate more portions of imaging surface than when approachmirror 140 is fixed. One example of such an embodiment is illustrated inFIG. 7.

FIG. 7 schematically illustrates two sets of scan lines on imagingsurface 110, according to an embodiment of the invention. With movableapproach mirror 140 in a first orientation, focused beam 175 follows afirst set 710 of scan lines (solid lines) on imaging surface 110. Aseach reflective facet of raster polygon mirror 150 rotates throughapproach beam 173, focused beam 175 follows one of scan lines 711-719.With movable mirror 140 in a second orientation, focused beam 175follows a second set 720 of scan lines (dashed lines) on imaging surface110. In the embodiment illustrated in FIG. 7, the scan lines of firstset 710 are interleaved with the scan lines of second set 720. Thus,movable approach mirror 140 can be used to increase the resolutionand/or size of an image produced by 2-D scanning system 100 withoutincreasing the number of lasers or other light sources in laser module120.

In some embodiments, an imaging system may include one or more foldingmirrors to provide a longer working distance between the scan andimaging lens and the imaging surface, improve the compactness of theimaging system, or both. FIG. 8 schematically illustrates an imagingsystem 800 that includes two folding mirrors 810, 820, according to anembodiment of the invention. Folding mirrors 810, 820 are positioned inthe optical path between scan optics 160 and imaging surface 110, andare configured to direct focused beam 175 to imaging surface 110.

As noted above, in some embodiments, 2-D scanning system 100 may includemultiple laser modules 120 that together produce a plurality ofsubstantially parallel laser beams, rather than a single laser beam 171as depicted in FIG. 2. For example, 2-D scanning system 100 may include5, 10, 20, or more laser modules, according to some embodiments of theinvention. In such embodiments, each of scan lines 301-309 illustratedin FIG. 3 and scan lines 401-409 illustrated in FIG. 4 represents a pathfollowed by a single laser beam, rather than the paths followed by alllaser beams generated by laser module 120. For example, in oneembodiment, each scan line illustrated in FIGS. 3 and 4 represents apath followed by the centermost laser beam generated by laser module120.

FIG. 9 sets forth a flowchart of method steps for determining theconfiguration of scan optics 160, according to embodiments of theinvention. Although the method steps are described with respect to 2-Dscanning system 100 of FIG. 2, persons skilled in the art willunderstand that performing the method steps to determine theconfiguration of a scan and imaging lens in any imaging system using araster-scanning polygon is within the scope of the invention. Prior tothe start of the method 900, a general configuration of 2-D scanningsystem 100 is determined, including the size of imaging surface 110 andthe relative positions of laser module 120, raster polygon mirror 150,and scan optics 160.

As shown, method 900 begins at step 901, where approach angle 141 isdetermined. Because a smaller approach angle 141 produces lessasymmetric distortion of scan lines on imaging surface 110, in someembodiments the location and orientation of approach mirror 140 isselected to create the smallest practical approach angle 141, given aspecific geometry of 2-D scanning system 100. The magnitude of approachangle 141 may be determined based on height 119 of imaging surface 110,the beam width 145 of collimated beam 172, and pupil distance 159between raster polygon mirror 150 and front components of scan optics160. In embodiments of the invention in which approach mirror 140 ispositioned between raster polygon mirror 150 and imaging surface 110,approach angle 141 may be about 30° to 45°.

In step 902, tilt angle 157 of rotational axis 156 is selected tooptimize distortion of laser scan lines followed by focused beam 175 onimaging surface 110. Specifically, based on approach angle 141, tiltangle 157 can be selected so that the distortion of scan lines onimaging surface 110 is symmetrical. In one embodiment, optical modelingsoftware known in the art can be used to predict scan-line shapes on ascreen, and, through ray-tracing, determine an optimal value of tiltangle 157 to position the pattern of pin-cushion distortionsymmetrically on imaging surface 110. Given approach angle 141 asdetermined in step 901, and ideal scan optics 160, one of ordinary skillin the art can readily determine such an optimal value of tilt angle 157using such a process. In such an embodiment, ideal scan optics 160 areassumed to be free of aberration.

In step 903, the degree of pin-cushion distortion introduced into 2-Dscanning system 100 by raster polygon mirror 150 is determined. Opticalmodeling software known in the art, such as a scan-line ray-tracingalgorithm, can be used to predict the scan-line shapes that are followedby focused beam 175 on imaging surface 110. In this way, symmetricpin-cushion scan-line distortion introduced into 2-D scanning system 100by raster polygon mirror 150 can be quantified. In embodiments in whichlaser module 120 produces multiple laser beams, scan lines correspondingto the path followed by one or more representative lasers may bepredicted in step 903, rather than predicting the scan lines for alllaser beams generated by laser module 120.

In step 904, scan optics 160 are configured to produce equal andopposite scan-line distortion in focused beam 175 to that predicted tobe present in 2-D scanning system 100 in step 903. Thus, scan optics 160are configured to produce a specific amount of negative, or barrel,distortion in focused beam 175 to compensate for the positive, orpin-cushion, distortion of focused beam 175 determined in step 903 to bepresent in 2-D scanning system 100.

FIG. 10 is an example of a ray-tracing scan-line diagram illustratingscan lines produced on a screen by embodiments of the invention. FIG. 10was generated with a scan-line ray-tracing macro over a 20″×15″ imagingscreen. As shown, laser scan lines across the screen are substantiallystraight and parallel lines rather than arcs. Specifically, embodimentsof the invention can achieve straightness accuracy of 0.5 mm over a 500mm long scan line on imaging surface 110, such as one of laser scanlines 401-409 in FIG. 4. Thus, embodiments of the invention can produceline straightness on an imaging surface using a single scanningcomponent to achieve a straightness error of 1/1000. The embodiment ofscan optics 160 is configured as an f-theta lens.

FIG. 11 schematically illustrates another embodiment 1100 of scan optics160 that is configured with a compensatory lens-distortion function,according to an embodiment of the invention. Similar to the embodimentof scan optics 160 illustrated in FIG. 11, the laser scan lines followedby focused beam 175 on imaging surface 110 are substantially straightlines due to the compensatory lens-distortion function. Similar to scanlenses known in the art, embodiment 1100 of scan optics 160 isconfigured to focus focused beam 175 on imaging surface 110 with minimalaberration at all points on imaging surface 110. Light beam 1150 isshown in FIG. 11 to qualitatively illustrate the behavior of light beamsthat pass through embodiment 1100 of scan optics 160 and are directed toimaging surface 110.

In embodiment 1100 illustrated in FIG. 11, scan optics 160 comprise afour-element compound lens that includes elements 1101-1104, where eachof elements 1101-1104 has a specific function. Taken together, thefunctions of elements 1101-1104 focus light beam 1150 on imaging surface110 with minimal aberration and with a compensating barrel distortionthat substantially cancels the pin-cushion distortion that is producedby other components of 2-D scanning system 100. In the embodimentillustrated in FIG. 11, elements 1101-1104 are each spherical elements,which are generally more manufacturable than aspherical opticalelements. In addition, scan optics 160 are symmetrically positioned withrespect to imaging surface 110, i.e., scan optics 160 are positionedsuch that a ray passing along the optical axis 1160 of scan optics 160also passes through a center point 1170 of imaging surface 110. Centerpoint 1170 is equidistant from top edge 1161 and bottom edge 1162 ofimaging surface 110 and is also equidistant from the left and rightedges (not shown) of imaging surface 110. Besides the advantagedescribed above for the embodiment of scan optics 160 illustrated inFIG. 5, embodiment 1100 only includes 4 elements, and is thereforeeasier-to fabricate and assemble. In addition, embodiment 1100 isgenerally more compact than embodiments of scan optics using five ormore elements.

Element 1101 is the first element of embodiment 1100 through whichreflected beam 174 passes. Element 1101 is configured to generateoptical power of an incident beam with minimal aberration. Element 1102is configured to compensate for on-axis aberrations introduced byElement 1101. Element 1103 is configured to compensate for off-axisresidual aberrations introduced by elements 1101 and 1102, such asastigmatism and field curvature. Element 1104 is configured mainly as acompensating distortion element that generates enough negative, i.e.,barrel, distortion to compensate for scan-line curvature of focused beam175 introduced by using raster polygon mirror 150 to scan focused beam175 on imaging surface 110. In some embodiments, element 1104 is apositive, or converging, lens. It is noted that embodiment 1100 of scanoptics is configured as an f-theta lens.

Given approach angle 141, pupil distance 159, effective focus distance169, (shown in FIG. 2) and the dimensions of imaging surface 110, one ofskill in the art, upon reading the disclosure herein, can readily devisea configuration of elements 1101-1104 having the functionality describedabove. In such a configuration, each of elements 1101-1104 may vary fromeach other in one or more optical characteristics, including firstsurface radius, second surface radius, element thickness, glass type,dispersion, relative position to adjacent elements, index of refraction,and entrance pupil location. In some embodiments, the configuration ofeach of elements 1101-1104, i.e., the above optical characteristics forelements 1101-1104, are determined simultaneously, since all fourelements work cooperatively to ensure proper focus and barrel distortionof light beams 1111-1104 on imaging surface 110.

In sum, embodiments of the invention set forth a scanning system thatuses only one quickly-rotating component, i.e., a raster-polygon, toproduce 2-D straight scan lines on an imaging surface. One advantage ofthe present invention is that a single rotational element can be used toachieve two-dimensional scanning of light onto an imaging surface withstraight and parallel scan lines.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

We claim:
 1. A two-dimensional scanning system, comprising: an imagingsurface; multiple light sources configured to generate multiple lightbeams, each beam for illuminating a distinct portion of the imagingsurface; a rotatable raster polygon that has multiple reflective facetsand is positioned in an optical path between the multiple light sourcesand the imaging surface to direct the multiple light beams to theimaging surface with the reflective facets, wherein the reflectivefacets are each inclined at a different angle with respect to arotational axis of the rotatable raster polygon; and a plurality of scanlenses positioned in an optical path between the rotatable rasterpolygon and the imaging surface and configured to introduce barreldistortion of multiple scan lines that the multiple light beams followacross the imaging surface as the rotatable raster polygon rotates,wherein each scan lens has a convex surface facing an imaging surface;wherein the rotational raster polygon is positioned at a tilt angleinclined toward the imaging surface; wherein an approach angle of themultiple beams is selected to position a convergence point of themultiple laser beams proximate a reflective facet of the rotatableraster polygon; wherein the multiple light beams reflected from a firstfacet of the rotatable raster polygon is along a first optical paththrough a first portion of the scan lens at a first incident angle andare incident on the imaging surface at a first vertical position;wherein the multiple light beams reflected from a second facet of therotatable raster polygon is along a second optical path through a secondportion of the scan lens at a second incident angle different from thefirst incident angle and are incident on the imaging surface at a secondvertical position different from the first vertical position; andwherein the magnitude of the approach angle is determined based on aheight of the imaging surface, the beam width of an approach beam, andthe pupil distance between a raster polygon mirror and a scan opticsmodule.
 2. The system of claim 1, wherein the imaging surface includes adisplay screen.
 3. The system of claim 2, wherein the display screen ison a side of the scan optics module opposite the rotatable rasterpolygon.
 4. The system of claim 3, wherein the display screen and thescan optics module are symmetrically positioned to each other.
 5. Thesystem of claim 2, wherein the center point of the scan optics moduleand the center point of the display screen are such that the centerpoints are equidistance from a top edge and a bottom edge of the displayscreen and are equidistant from a top edge and a bottom edge of the scanoptics module pupil and are also equidistant from left and right edgesof the scan optics pupil.
 6. The system of claim 1, wherein theplurality of scan lenses consists of spherical lens elements.
 7. Thesystem of claim 6, wherein the plurality of scan lenses are positionedin the optical path between the rotatable raster polygon and the imagingsurface such that a light ray directed along an optical axis of theplurality of scan lenses is incident on a center point of the imagingsurface that is equidistant from a left edge and a right edge of theimaging surface and equidistant from a top edge and a bottom edge of theimaging surface.
 8. The system of claim 6, wherein the plurality of scanlenses comprises five-element lenses.
 9. The system of claim 8, whereinthe five-element lenses are configured as a compensating distortion lensto generate the barrel distortion of the multiple scan lines.
 10. Thesystem of claim 6, wherein the plurality of scan lenses comprisesfour-element lenses configured as a compensating distortion lens togenerate the barrel distortion of the multiple scan lines.
 11. Thesystem of claim 1, wherein the barrel distortion introduced by theplurality of scan lenses compensates for pin-cushion distortion of themultiple scan lines such that the multiple scan lines are substantiallystraight and parallel lines across the entire imaging surface.
 12. Thesystem of claim 1, further comprising a reflective element disposed inan optical path between the multiple light sources and the rotatableraster polygon, wherein the reflective element is positioned closer tothe imaging surface than the rotatable raster polygon is positioned tothe imaging surface.
 13. The system of claim 12, wherein the reflectiveelement is configured to move between a first orientation that directsthe multiple light beams along a first set of scan lines on a firstportion of the imaging surface as the rotatable raster polygon rotatesand a second orientation that directs the multiple light beams along asecond set of scan lines on a distinct second portion of the imagingsurface as the rotatable raster polygon rotates.
 14. The system of claim13, wherein scan lines of the first set of scan lines are interleavedbetween scan lines of the second set of scan lines.
 15. The system ofclaim 14, wherein the tilt angle is selected so that a pin-cushiondistortion pattern produced by the multiple scan lines on the imagingsurface is positioned symmetrically so that a centerline of thepin-cushion distortion pattern is substantially aligned with thecenterline of the imaging surface.
 16. The system of claim 12, whereinthe barrel distortion of multiple scan lines that the multiple lightbeams follow across the imaging surface causes substantially straightand parallel imaging surface light beam scan lines.
 17. The system ofclaim 1, further comprising a folding mirror disposed in an optical pathbetween the plurality of scan lenses and the imaging surface andconfigured to direct the multiple light beams to the imaging surface.18. The system of claim 1, wherein the plurality of scan lenses arepositioned from the rotatable raster polygon by a pupil distance andfrom the imaging surface by an effective focus distance and theeffective focus distance is greater than the pupil distance.
 19. Thesystem of claim 1, wherein the plurality of scan lenses comprisesfive-element lenses.
 20. The system of claim 1, wherein the plurality ofscan lenses comprises four-element lenses configured as a compensatingdistortion lens to generate the barrel distortion of the multiple scanlines.